The goal of the proposed experiments is to understand the mechanisms that control sensory information processing in the cerebral cortex of adults mammals. Previous results by Armstrong- James and Fox, included in this proposal, have quantified the receptive field of cortical neurons in the barrel field cortex representing the mystacial vibrissae by measuring their response to vibrissal deflections. An important conclusion from these studies is that layer IV neurons that responded with the highest probability (2 or more spikes per stimulus over 50 repetitions) to stimulation of their appropriate vibrissa also responded at short latencies (<10 msec). In addition to the best response (here called the Center Receptive Field or CRF), layer IV neurons also showed lower probability, longer latency to onset of discharge to an additional number of whiskers (2.6 on average in the Excitatory Surround or SRF). Two hypothesis will be tested by the proposed experiments. One is that the high probability (>2 spikes/stimulus) + short latency (<10 msec) responses of barrel field cells to vibrissae deflection are generated through the VB pathway to cortex, while the lower probability (1 or fewer spikes/stimulus) + longer latency (>10 msec) responses are generated either through the spinal V to POM pathway or through corticocortical connections; either through SII cortex or short intracortical connections within SI or both. The second hypothesis is based on the observation that some components within SI cell activity depend upon mechanisms that require activation of acidic amino acid receptors, specifically the NMDA receptors. NMDA receptors have been shown to be essential for the generation of spontaneous burst activity, but not for the activation of cortical neurons by sensory inputs. Based on this differential dependence on the NMDA receptors, we predict that these receptors will be necessary for generating the surround, but not the center component of the layer IV cortical cell receptive field responses. Evidence supporting these hypotheses would provide new insights into the neural circuitry underlying the receptive fields of cortical neurons and could identify important functional interactions between the specific and the nonspecific thalamocortical projection systems that may regulate the strength of synapses in sensory cortex.